1. Field of the Invention
The present invention relates to ferroelectric semiconductor devices; and more particularly, to a thin film material, method of deposition, and devices incorporating same. Although, the thin film material and method of deposition of the present invention is suitable for many destructive read-out (DRO) and nondestructive read-out (NDRO) applications, it is particularly advantageous for use in very large scale integrated circuits (VLSIC) having a ferroelectric storage medium on a semiconductor substrate, such as a ferroelectric (NDRO) field effect transistor (FEMFET) and a capacitive (DRO) memory, for example, and will be described in more detail in connection therewith.
2. Discussion of Related Art
Information processing and memory technologies remain one of the fastest growing and most rapidly changing areas in VLSIC. For example, future avionics systems will be required to process and transfer massive amounts of data from on-board and remote sensors at very high speeds. Many of the memory technologies now installed in field equipments or space vehicles are out of production and only available at a premium price. Core memories are used in tactical computers because of the need for non-volatility and radiation hardness. The slow access time and low packaging density of core memories impose limitations on speed and capacity in military computers. Meanwhile, increasingly high speed and capacity are required for the more sophisticated signal processing of modern systems. Plated wire memories are used in strategic missiles; and they are heavy, bulky, and consume a considerable amount of power. This adds to the propulsion requirements of the missile. Core and plated wire memories are old technologies that need replacement by a technology which incorporates decoding and amplification on the same chip. The ferroelectric RAM (FERRAM) is such an integrated memory, and it is fabricated using low-cost photolithographic techniques. A nonvolatile, NDRO, high density computer memory technology which can be radiation hard and which has both write (program) and access times on the order of 50 ns or less, with 5-volt programming is needed.
In order to meet these needs, thin films of ferroelectric materials have been proposed for memory applications. Among these proposals are the nonvolatile ferroelectric random access memory (RAM) chips using thin dielectric films, such as lead zirconium titanate (PZT) integrated memory structures involving KNO.sub.3 thin film capacitors, and optically addressed memory arrays based upon thin films of composition Pb.sub.0.92 Bi.sub.0.07 La.sub.0.01 (Fe.sub.0.405 Nb.sub.0.325 Zr.sup.0.27)O.sub.3, designated PZBFN-65. Among the more recent developments are the ferroelectric random access memory (RAM) chips using thin films of ceramic lead lanthanum zirconium titanate (PLZT).
A potentially important memory cell embodiment is the ferroelectric memory transistor (FEMFET), which utilizes a ferroelectric gate dielectric grown directly on a bulk semiconductor, such as silicon. Previous efforts demonstrated that structures of this type incorporating bismuth titanate films could be grown on silicon, and yielded devices exhibiting high speed and high retentivity. However, it appeared that conductivity modulation was dominated by the mechanism of carrier injection from the semiconductor into the SiO.sub.2 /ferroelectric interface traps, especially at low gate signal frequencies.
It is well known, that the ferroelectric properties of thin films grown on foreign substrates are strongly affected by defects, grain boundaries, and deviation from proper stoichiometry. The defects not only affect the ferroelectric behavior, but also the electronic properties. One common problem with the ferroelectric memory materials currently in use is that they suffer from fatigue (reduction of the magnitude of the remanant polarization after repeated cycling operations) and aging (decrease in the material polarization due to fewer domain realignments with constant poling fields) phenomena, thus making them unreliable for these applications. For memory applications, it is preferred that these films be grown on silicon or some other semiconductor substrate for integration with associated electronic circuits. In the fabrication of such devices processing temperatures, which are too high can have deleterious effects on previously deposited films. Also, low rates of deposition can make the devices costly to manufacture.